Electrical resistor with a polycrystalline ceramic cover and a process for its manufacture

ABSTRACT

A new covering material for electrical resistors and the related process for the manufacturing of these resistors are disclosed, according to which an insulating and protective vitreous polycrystalline ceramic is obtained by controlled in-situ crystallisation of vitreous material so as to form microcrystals uniformly dispersed in the vitreous mass, the above mentioned crystallisation being carried out in the vitreous starting material prepared by powdering mixtures adapted for producing vitreous substances, already covering the ceramic support of the resistor already fitted with the resistive winding and the rheophores, by baking the assembly at a temperature of 600* 850* C for 5 - 120 minutes. The invention relates also to the new class of insulated resistors thus obtained.

[111 3,919,682 [451 Nov. 11, 1975 [54] ELECTRICAL RESISTOR WITH A POLYCRYSTALLINE CERAMIC COVER AND A PROCESS FOR ITS MANUFACTURE [75] Inventor: Ubaldo Costa, Milan, Italy [73] Assignee: S.E.C.I." Soc. Elettrotecnica Chimica Italiana S.p.A., Milan, Italy [22] Filed: Sept. 4, 1973 211 Appl. No.: 393,998

[30] Foreign Application Priority Data Sept. 8. 1972 Italy 28983/72 [56] References Cited UNITED STATES PATENTS 1/1966 Berkelhamer 338/266 7/1968 Carman 106/54 OTHER PUBLICATIONS Dummer Fixed Resistors, Pitman London, 1967,

Primary E.ranziner-E. A. Goldberg Attorney, Agent, or Firnz-Diller, Brown, Ramik & Wight [5 7] ABSTRACT A new covering material for electrical resistors and the related process for the manufacturing of these resistors are disclosed, according to which an insulating and protective vitreous polycrystalline ceramic is obtained by controlled in-situ crystallisation of vitreous material so as to .form microcrystals uniformly dispersed in the vitreous mass, the above mentioned crystallisation being carried out in the vitreous starting material prepared by powdering mixtures adapted for producing vitreous substances, already covering the ceramic support of the resistor already fitted with the resistive winding and the rheophores, by baking the assembly at a temperature of 600 850C for 5 120 minutes. The invention relates also to the new class of insulated resistors thus obtained.

13 Claims 8 Drawing Figures U.S. Patent Nov.1l, 1975 Sheet10f3 3,919,682

emperature Differential I l I l 200 400 600 800 1000 1200 Temperature (C Diffe rential Temperature Temperature (C) U.S. Patent Nov. 11, 1975 Sheet20f3 3,919,682

35 mins.

US. Patent N0v.11, 1975 Sheet3of3 3,919,682

6'0 '70 QCTmins.

Fig.7

ELECTRICAL REslsToR WITH A POLYCRYSTALLINE CERAMIC COVER AND A PROCESS FOR ITS MANUFACTURE NiCr, constantan-etc.) is wound on a ceramic supportv generally of cylindrical, flat or tubular shape and is coveredvvith a number of layersof a vitreous enamel which is baked at temperatures greater than 850C.

In this manner the entire resistive winding becomes embedded in a moreor less thick layer of enamel.

The vitreous enamel of these resistors must have a coefticient of thermal expansion which issuitable for the cerami e support so that the individual materials (ceramic support enamel and metal materials) are coupled withoutir nechanical-tension and the formation of cracks in the ,vitreouser amel-is avoided even if the resistor is :subjectedtoalternate variable, repeated cycles off heating olr cooling, As stated, during the various stages of manufacture .the resistors undergo various. baking cycles at hightemperatures usually greater than Under these conditions the NiCr alloy, of which the resistive element is usuallymade, becomes subjected a number of times to temperatures at whichthe crystals of the alloy undergo coarsening. Thisphenomenon of crystallisation frequently causes breakage of the resistive element,Lespeciallywhen,theNiCr wire has a diam eter less than 'lzhe'resistive element may thus break not oril'y during the.,rrianufacturing process but also during'operatilon of the resistor, because the elastic characteristics and mechanical strength of the alloyare worsened after exposure to various cycles of high temperature, precisely because of the phenomenon of coarsening of the crystals and also because. of the. mechanical forces" which arise'in t'he mass of the enamel, especially when it iscrack'ed. v

To reduce or eliminate these grave disadvantagegattempts have been made to util ise vitreous enamels applicable at temperatures lower'than 800- 850C, but i the vitreous: enamels hs'ed u pto the" present timehave been found unsuitable for resolving this problem. In

fact, it is well' knoivn' that vitre'ous enamels, which are substantially amorphous glasses to which some colouring pigment 's'uch"as "Cr O C00 etc. have been added, i

have coefficients of expansion whichare higher the lower their softening'point.

The attempts ma d e' 'u p to the present time of modify- I I ing the composition of'these vitreous enamels for the purpose of loweringtheir softening point and hence their baking" temperature while maintaining a suitable A coefficient ofexpansion have not met with success.

When one tries to reduce the aforementioned disadvantages by using vitreous enamels having temperatures of application lower than 850C, i.e. forexample between 0" C and 800C, the resistors prove to be of I very poor quality'b'ecaii'se of'the vitreous enamel being badlyeracked due to'its coefficient of expansion being no longer suitable for the type of ceramic supports commercially available.

Furthermore it must be added that vitreous enamels of low baking temperature have a poor mechanical strength, a low viscosity at the baking temperature of the resistor and a higher chemical reactivity.

The low mechanical strength of these vitreous enamels leads to a greater ease of formation of cracks. A low viscosity at the baking point of the resistor aids the formation of bubbles and hence of holes in the enamel by imprisoningair, in addition to making the production of these resistors difficult in that the enamel tends eas ily to 1 flow during baking so causing malformation which makes the resistor unuseable.

During the manufacturing process for enamelled're sistors a certain percentage of rejects is due to the movement of the' 'resistive wire in that .as the vitreous enamel becomes liquid with alow viscosity it cannot prevent the natural movement of the resistivewire in consequenceof ite W coefficient of expansion. This causes someturns of the resistive winding to come into contact with, each other, so leading to large variations in ohmic value during manufacture.

The geometrical shapeof the enamelled resistor, precisely because the vitreous enamel behaves as a liquied at the baking temperature of the resistor, is not always perfectly uniform so that on the resistor there are points having different thicknessesof enamel with consequent different values of insulation resistance and dielectric strength. A For these reasons, i.e., the presence of bubbles, holes and different enamel thicknesses, it is practically impossible to guarantee the insulation of enamelled resistors produced with. vitreous enamels having a low softening temperature v I Furthermore the high chemical reactivity of the vitreous enamel more easily gives rise to reaction with metal .materialsof which the resistor is constituted (wires, rheophores etc.) and this leads to the formation of spongy zones. It is consequently practically impossible with vitreous enamels of low softening point known up to the present time toobtain a enamelled wire resistor of good quality and with a high level of reliability.

Consequently the objectof the present invention is to provide a covering for electrical wire resistors which enables the aforementioned disadvantages to be obviated. I A further object is to' provide a new material for covering resistors which has excellent characteristics of uniformity of s tructure,.mechanical, strength, insulating resistance and dielectric strength, and moreover having a coefficient of expansion which makes it particularly suited to covering resistors.

A further object is to provide a method for preparing the resistors covered with the new covering.

These and further objects are obtained according to the present invention, if a resistor consisting of a ceramic support of any type or shape, with a resistive element of wire or suitable metal alloy strip (such as Ni- Cr, constantan etc.) and provided with rheophores of suitable material (for example 42 alloy, Fe-Cr alloy etc.) is covered with an insulating and protective inorganic material namely vitreous ceramic and defined as a polycrystalline ceramic obtained by controlled in-situ crystallisation of a vitreous material in such a manner the vitreous mass; this vitreous ceramic, after applicavitreous substances, which consist mainly of SiO,, 8

and Pb O powder and possible other minor components in the presence of nucleants and/or crystallisation promoters in the form of powders, such as titanium dioxide -of the rutile type, chromium oxide etc.

By heating this mixture until it melts, at approximately l=300C, a so-called frit is obtained, i.e., a substance consisting essentially of a vitreous material. Crystallisation can be controlled in-situ during the annealing of the material so obtained by adopting a suitable. baking cycle, and depending on thequantity and type of nucleant used.

4 tice is less than that of the liquid state because of which a temperature rise takes place in the mass.

Fusion of the crystal on the other hand gives rise to an endothermic effect and hence to a lowering of temperature.

FIG. 1 shows a typical differential thermal analysis diagram for a vitreous ceramic.

The amorphous vitreous phase can represent up to 95% of the vitreous ceramic, or the polycrystalline phase may represent 95% of the product.

The vitreous ceramics may also be prepared by adding nucleants and/or crystallisation promoters to powdered glass and then baking the mixture obtained under suitable thermal cycles.

The presenceof micro-crystals uniformly distributed in the mass of the vitreous ceramic material creates a new type of protective material which has different characteristics than the original glass.

In particular, this new material defined as vitreous ceramic which is formed during the baking of the resistor has a high uniformity and a structure practically free from internal pores, high mechanical strength, high insulation resistance and high dielectric strength.

When conventional vitreous enamels exceed the softening temperature they rapidly become fluid for small temperature increases, whereas vitreous ceramic materials have the interesting and very useful characteristic of maintaining their viscosity practically constant even at temperatures very much higher than the softening temperature.

Not only is the coefficient of expansion of this material not related to its softening temperature, but it may be varied independently of it and is generally of the order of l.l0-8.l0

It is'thus possible to form a material having a low coefficient of expansion and a low softening temperature, even less than 800C; this is something, as already stated, which cannot be obtained with amorphous vitreous enamels.

The coefficient of expansion of a vitreous ceramic is lower than that of the vitreous phase present in it because the crystals which form have a lower coefficientof expansion than that of the glass in which they are immersed, and hence as the process of formation of the crystals can be controlled, it is possible to reduce this coefficient.

Physically,'these vitreous ceramic materials are distinguished from the well known amorphous vitreous enamels and divitrifiable glasses mainly because they have a microcrystalline structure finely and uniformly distributed in a vitreous matrix.

Changes of structure from amorphous to crystalline can be revealed by different thermal analysis in that they are accompanied by the development or absorption of energy in the form of heat.

When a substance crystallises, an exothermic effect is produced because the free energy of the crystalline lat- At point A there is a softening of the vitreous phase, at points B and C the formation of crystals (temperature increase), at point D the fusion of the crystalline phase (lowering of temperature).

The same diagram for a vitreous enamel (as shown in FIG. 2) only shows the softening point ofthe single vitreous phase present.

Naturally the crystalline phase of a vitreous ceramic is clearly visible and analysable by X-ray diffraction, whereas an amorphous enamel is completely transparent to X-rays.

In comparison with normal vitreous enamels, vitreous ceramics adhere very well to the ceramic supports and metal materials constituting the resistor.

This latter characterisic is particularly useful and important in the manufacture of wire resistors in that the vitreous ceramic protectionis chemically and physically well bonded with all the parts which constitute the resistor including the metal outlet rheophores.

Wire resistors of any shape and size protected by the vitreous ceramic covering materials of the present invention constitute a new class of resistors capable of supporting the most severe conditions of use, such as those which no other wire resistor protected by the normal amorphous vitreous enamels known today can support.

Considering now in particular the process for manufacturing wire. resistors covered with the new vitreous ceramic material of the present invention, this comprises in substance the operations of applying a layer of the initial vitreous material already containing the nucleants and/or crystallisation promoters to the ceramic support previously fitted with the resistive winding and present invention, the variouscomponents of the final vitreous ceramic, i.e. vitreous components, nucleants and/or crystallisation promoters in the form of fine powder uniformly mixed, are brought to a temperature of l,200l ,300C and the fused material is poured into water. The product of fusion is wet ground until at least 60% of the particles have a diameter less than 50. mi

cron and is then dried, granulated and then applied to the-ceramic support previously fitted with the resistive element and rheophores, by mechanical pressing, after which the above thermal cycle is'carried out, which produces the required vitreous ceramic covering.

In a further embodiment of the process of the present invention, an aqueous suspension is prepared from the wet ground product of fusion, with the possible addition of filling materials, and the already fittedsupport is covered by immersing it, then proceeding to the baking ,cycle in order to obtain the vitreous ceramic covering in-situ. Finally in a third embodiment the covering of the ceramic support prior to the thermal cycle is made in the form of a drawn tube which is mounted over the support carrying the resistive elements and rheophores, after which the thermal baking cycle is carried out.

If the covering is made by immersion or spraying, the covering operation and subsequent thermal treatment may be repeated a number of times, according to the required covering thickness and for ensuring homogeneity. v

The nucleisation and formation of crystals and their consequent coarsening begins as soon as a temperature of 700C is reached and proceeds more or less swiftly according to the temperature chosen between 600 and 850C. Crystallisation may be interrupted by cooling to below 600C.

By utilising this characteristic, the contents and size of the microcrystals may be dosed at will simply be acting on the temperature and/or on the baking time, and by suitably metering the quantity of nucleants and/or crystallisation promoters in the original glass formula.

The wire resistors protected by vitreous ceramic materials may be manufactured at a lower cost than those protected by normal well known vitreous enamels, because the baking temperatures of the vitreous ceramic material may be considerably lower which reduces the percentage of rejects during manufacture and also the time required for baking the resistors may be much shorter.

FIG. 3 is a partially sectional view of a resistor according to the present inventin, comprising a support of ceramic material 1, on which a resistive" element 2 is wound. The resistor is completed by the rheophores 3 connected to the ends of the resistive element 2, and the finished vitreous ceramic insulating covering is indicated by the reference numeral 4.

The wire resistors protected by vitreous ceramic have a greater reliability of operation over longperiods of time than resistors protected by'normal vitreous enamels because of the greater resistance to thermal shock, higher mechanical strength, lower thermal stress of the resistive elements during manufacture, higher chemical resistance, excellent adherence of the protective vitreous ceramic covering to the parts constituting the resistor (and particularly to the metal parts) and the absence of cracks.

material so enabling insulated wire resistors to be made whose insulating and protective material is chemically and physically bonded with the other parts constituting 4 and EXAMPLE 1 The following example illustrates the preparation of a vitreous ceramic according to the invention. A mixture is prepared consisting of the following powders (the percentages being expressed in weight in the total mixture):

SiO 15.20% B 0 32.00% ZnO 41.27% Na O 1.60% CaO 1.96% K 0 1.96% C00 215% C 0; 3.86%

and is brought to a temperature of 1200-l300C in a muffle furnace. The material in the fused state is then poured into water, and in this manner in addition to being cooled it is also reduced to grains of a diameter which varies between 1 mm and 10 mm. The material so obtained is then wet ground in a ball mill until a fine suspension of powder is obtained having a particle diameter less than 50 micron.

100 g. of the suspension so obtained were dried at 1002C and the dry powder was treated in accordance with the baking cycle shown in FIG. 4 so as to form insitu a homogeneous controlled distribution of dispersed microcrystals of the original glass. The material so formed may be defined as vitreous ceramic and has a coefficient of expansion measured between C and 370C of 510-.

constructed with a vitreous ceramic material consists of a ceramic supporting body 1 of steatite, foe example the type 22lK produced by Rosenthal Stemag Technische Keramik G.m.b.H. The rheophores 3 of iron-nickelmetal alloy and the rersistive element 2 were applied to this support by known methods. The protective covering for the resistor is prepared starting from the following composition:

SiO, 15.6% Al,0; 4.4% PbO 68.9% TiO, 1 l l The various components in the form of fine powder intimately and uniformly mixed are brought to a temperature between 1200 and 1300C. The fused material is then poured into water. The product of fusion reduced to grains is wet ground in a ball mill until 60% of the particles are less than about 50 micron. After drying, the powder is granulated using normal known procedures and then applied to the resistor by mechanical pressing.

The resistor so protected was brought to a temperature of 650C for 30 minutes and the external geometrical shape remained nearly unchanged. The finished resistor comprises a covering of a vitreous ceramic material obtained in-situ, consisting of microcrystals uniformly dispersed in the vitreous mass.

An X-ray analysis of the covering of this resistor showed the presence of crystalline products as shown in the diffractogram of FIG. 5.

EXAMPLE 3 An electrical resistor constructed with a vitreous ceramic material was made principally starting from a ceramic supporting body of steatite, for example the type 221K- of Rosenthal Stemag Technische Keramik G.m.b.I-I. The rheophores of iron-nickel metal alloy and the resistive element were applied to this support using known methods.

A protective covering with the composition referred to in example I was applied to the thus assembled resistor.

The aqueous suspension of the material described in Example 1 has the following composition: fine 50 micron powder of material described in Example 1, 70 g; clay 2 g; water 28 g.

The resistor is immersed in the described suspension so as to obtain a uniform covering. After drying, the therrrialtreatment according to the cycle shown in FIG. 6 is carried out. The application of the described aqueouss'uspension with consequent baking was repeated three times. The finished resistor comprised a covering of a vitreous ceramic material obtained in-situ, consisting of microcrystals uniformly dispersed in the vitreous mass.

The layer of vitreous ceramic covering the resistor,

' reduced to powder and subjected to an X-ray diffraction analysis, gave the diffractogram shown in FIG. 7.

EXAMPLE 4 An electrical resistor is constructed from a vitreous ceramic material as described in examples 1 and 2. The protective covering of the resistor is prepared starting from the following composition:

SiO l.4.60% B 30.40% ZnO 39.70% Na O l .50% C210 l .80% K 0 l .80% Al O 0. l 53 C00 2. l 2 o C r 0 3.5 l% TiO, 4.42%

The described composition was brought to a temperature of l200-l 300C so as to melt it. The fused material was poured into water and was reduced to grains and then wet ground in a ball mill until the particles of the material so obtained had a diameter of less than 10 micron.

The powder obtained was dried and made into a paste with a solution of 10% of methylcellulose in water.

The paste obtained was drawn in order to form tubes of suitable dimensions.

The tubes of suitable length were mounted over the resistors, the resistor was then baked first at a temperature of 400C for one hour in order to eliminate the methylcellulose, then at 780C for 30 minutes in order to produce the controlled crystallisation of the vitreous I port, a resistive element on said support, and conductors connected to said resistance element and extending from said support, and an inorganic insulating and protective cover chemically and physically bonded to all the parts constituting the resistor including those portions of said conductors adjacent said support, said cover comprising a polycrystalline ceramic obtained by controlled in situ crystallization of a vitreous material in such a manner as to obtain microcrystals uniformly dispensed and distributed in the vitreous mass.

2. An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover consists for of a vitreous phase and for 5% of a crystalline phase.

3. An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover consists for 95% of a crystalline phase and for 5% of a vitreous phase.

4. An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover has a coefficient of expansion lying between 1.10 and 8.10 and a softening point lying between 600 and 800C. 7

5.' In the method of making electrical resistor components which comprises the steps of:

a. forming a length of electrically resistive metal around a ceramic support member and electrically connecting conductor elements to the opposite ends of said length of electrically resistive metal to provide an uncovered resistor component subassembly; and then b. applying a protective covering around at least that portion of the resistor component sub-assembly which encompasses said electrically resistive metal and baking such protective covering at an elevated temperature to provide an integrated assembly; the improvement which comprises:

in step (b), providing said covering in the form of a vitreous ceramic frit which consists essentially of a mixture of ceramic oxides having a softening point and at least one crystalline phase within the temperature range of 600-850C, and wherein said baking is carried out in the temperature range of 600-850C for a time sufficient to convert at least 5% of saidfrit to crystalline phase.

6. The method of claim 5 in which the vitreous ceramiccovering is applied to the electrically resistive metal by pulversing it and then mechanically pressing the powder, possibly in the presence of known organic or inorganic binders such as methylcellulose, clay or the like and baking the covered electrically resistive metal at 600800C for a time of l minutes, so as to form in-situ the vitreous ceramic covering which constitutes the electrical insulation and protection of the electrically resistive metal.

7. The method of claim 5 in which the vitreous ceramic covering is provided in the form of powder which and the assembly is baked at said temperatures of 600-850C for 1-120 minutes so as to form in-situ the vitreous ceramic covering which constitutes the electrical insulation and protection for the electrically resistive metal.

8. The method of claim 5, in which the original electrically resistive metal is'immersed in a suspension in water of a fine powder consisting of the vitreous ceramic covering material containing nucleants and/or crystallisation promoters and is baked at said temperatures between 600 and 850C for 1-120 minutes so as to obtain in-situ on the electrically resistive metal the formation of the vitreous ceramic covering, the thermal cycle and application of the syspension being repeated at least twice so as to form on the electrically resistive metal a uniform thickness of vitreous ceramic material.

9. The method of claim 5, in which the original electrically resistive material is in the form of a wire which is sprayed with an aqueous suspension of a fine powder consisting of the vitreous ceramic covering material containing nucleants and/or crystallisation promoters and is baked at said temperatures between 600 and 850C for 1-120 minutes so as to obtain the vitreous ceramic covering in-situ on the wire, the thermal cycle and spray application of the aqueous suspension on the wire being repeated at least twice so as to form a uniform thickness of vitreous ceramic covering on the wire.

10. In the method according to claim 5 wherein said frit consists essentially-of the following weight percentages:

LII

SiO 10-28% B 0 26-50% ZnO 28-40% Na O 05-10% K 0 05-10% A1 0 0.0l-l4% CaO 0.0 1-4% C0 0 05% Cr O 0-57: TiO 0-1 0%,

the chromium and titanium oxides being mutually exclusive.

11. In the method according to claim 5 wherein said frit consists essentially of the following:

SiO about 152% 8,0; about 32.0% ZnO about 41.27% Na O about 1.6% CaO about 1.96% K 0 about 1.96% CoO about 2.15% Cr O about 3.86%,

the percentages by weight.

12. In the method according to claim 5 wherein said frit consists essentially of the following:

SiO about 15.6% A1 0 about 4.4% P 0 about 68.9% TiO, about 11.1%,

then wet grinding the material to form the frit. 

1. An electrical resistor comprising a ceramic support, a resistive element on said support, and conductors connected to said resistance element and extending from said support, and an inorganic insulating and protective cover chemically and physically bonded to all the parts constituting the resistor including those portions of said conductors adjacent said support, said cover comprising a polycrystalline ceramic obtained by controlled in situ crystallization of a vitreous material in such a manner as to obtain microcrystals uniformly dispensed and distributed in the vitreous mass.
 2. An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover consists for 95% of a vitreous phase and for 5% of a crystalline phase.
 3. An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover consists for 95% of a crystalline phase and for 5% of a vitreous phase.
 4. An electrical resistor as claimed in claim 1, in which the vitreous ceramic insulating and protective cover has a coefficient of expansion lying between 1.10 6 and 8.10 6 and a softening point lying between 600* and 800*C.
 5. In the method of making electrical resistor components which comprises the steps of: a. forming a length of electrically resistive metal around a ceramic support member and electrically connecting conductor elements to the opposite ends of said length of electrically resistive metal to provide an uncovered resistor component sub-assembly; and then b. applying a protective covering around at least that portion of the resistor component sub-assembly which encompasses said electrically resistive metal and baking such protective covering at an elevated temperature to provide an integrated assembly; the improvement which comprises: in step (b), providing said covering in the form of a vitreous ceramic frit which consists essentially of a mixture of ceramic oxides having a softening point and at least one crystalline phase within the temperature range of 600*-850*C, and wherein said baking is carried out in the temperature range of 600*-850*C for a time sufficient to convert at least 5% of said frit to crystalline phase.
 6. The method of claim 5 in which the vitreous ceramic covering is applied to the electrically resistive metal by pulversing it and then mechanically pressing the powder, possibly in the presence of known organic or inorganic binders such as methylcellulose, clay or the like and baking the covered Electrically resistive metal at 600*-800*C for a time of 1 - 120 minutes, so as to form in-situ the vitreous ceramic covering which constitutes the electrical insulation and protection of the electrically resistive metal.
 7. The method of claim 5 in which the vitreous ceramic covering is provided in the form of powder which is dried, mixed with suitable known crystallisation agents and/or nucleants and binders to form a paste, the paste is drawn into tubes of suitable length, the tubes are mounted over the electrically resistive metal, and the assembly is baked at said temperatures of 600*-850*C for 1-120 minutes so as to form in-situ the vitreous ceramic covering which constitutes the electrical insulation and protection for the electrically resistive metal.
 8. The method of claim 5, in which the original electrically resistive metal is immersed in a suspension in water of a fine powder consisting of the vitreous ceramic covering material containing nucleants and/or crystallisation promoters and is baked at said temperatures between 600* and 850*C for 1-120 minutes so as to obtain in-situ on the electrically resistive metal the formation of the vitreous ceramic covering, the thermal cycle and application of the syspension being repeated at least twice so as to form on the electrically resistive metal a uniform thickness of vitreous ceramic material.
 9. The method of claim 5, in which the original electrically resistive material is in the form of a wire which is sprayed with an aqueous suspension of a fine powder consisting of the vitreous ceramic covering material containing nucleants and/or crystallisation promoters and is baked at said temperatures between 600* and 850*C for 1-120 minutes so as to obtain the vitreous ceramic covering in-situ on the wire, the thermal cycle and spray application of the aqueous suspension on the wire being repeated at least twice so as to form a uniform thickness of vitreous ceramic covering on the wire.
 10. In the method according to claim 5 wherein said frit consists essentially of the following weight percentages:
 11. In the method according to claim 5 wherein said frit consists essentially of the following:
 12. In the method according to claim 5 wherein said frit consists essentially of the following:
 13. In the method according to claim 10 wherein said frit is prepared by melting the ingredients at a temperature of 1200*-1300*C, pouring the melt into water and then wet grinding the material to form the frit. 